1. Field
The present disclosure relates to a light-emitting device including a light-emitting element and a wavelength conversion member, and an image display apparatus including the light-emitting device.
2. Description of the Related Art
In recent years, light-emitting devices including combinations of (i) light-emitting elements, such as light emitting diodes (LEDs), and (ii) wavelength conversion members (for example, members containing phosphor particles dispersed in resins) that convert excitation light from the light-emitting elements into phosphorescence have been developed. Such light-emitting devices have small size and lower power consumption than those of incandescent lamps. Thus, such light-emitting devices have been practically used as light sources for various image display apparatuses and lighting equipment.
As such light-emitting devices, combinations of blue LEDs and yellow phosphors are commonly used. As yellow phosphors, Ce-activated yttrium-aluminum-garnet (YAG) phosphors are widely used because of its high luminous efficiency.
In the case where a light-emitting device is used as a backlight for an image display apparatus, the color reproduction range of the image display apparatus is broadened as the half width of an emission spectrum of a phosphor decreases. However, the emission spectrum of a Ce-activated YAG phosphor has a relatively large half width of about 100 nm. In the case where a semiconductor light-emitting device including the Ce-activated YAG phosphor serving as a yellow phosphor is used as a backlight for a liquid crystal in an image display apparatus, the color reproduction range is not sufficiently wide.
Specifically, the image display apparatus covers almost the entire range of the color gamut of sRGB, which is used for cathode-ray tubes (CRTs). However, the image display apparatus has low coverage of the Adobe RGB color gamut, which is used for wide-color-gamut liquid crystal displays.
More specifically, the color gamut of the image display apparatus including the semiconductor light-emitting device which contains the Ce-activated YAG yellow phosphor and which serves as a liquid crystal backlight has about 70% coverage of the Adobe RGB color gamut. Thus, the semiconductor light-emitting device is not suitably used for wide-color-gamut liquid crystal displays.
The color gamut of sRGB used here indicates a color gamut represented by a triangle defined by three chromaticity points: (CIEx, CIEy)=(0.640, 0.330), (0.300, 0.600), and (0.150, 0.060) on Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates.
Meanwhile, the color gamut of Adobe RGB indicates a color gamut represented by a triangle defined by three chromaticity points: (CIEx, CIEy)=(0.640, 0.330), (0.210, 0.710), and (0.150, 0.060) on the CIE 1931 chromaticity coordinates. A comparison between the color gamut of sRGB and the color gamut of Adobe RGB reveals that the color reproduction range of green in the color gamut of Adobe RGB is larger than that of sRGB.
A semiconductor light-emitting device used as a backlight for a wide-color-gamut liquid crystal display corresponding to the Adobe RGB color space may have a structure in which two phosphors, i.e., a green phosphor and a red phosphor, are used in combination. The half width of each of the phosphors may be narrow.
For example, International Publication No. 2009/110285 (publication date: Sep. 11, 2009) and Japanese Unexamined Patent Application Publication No. 2010-93132 (publication date: Apr. 22, 2010) each disclose a semiconductor light-emitting device including a combination of a Eu-activated β-SiAlON phosphor and a Mn4+-activated fluoride complex. The combination enables an image display apparatus to have a wide color reproduction range, compared with a conventionally typical structure including a yellow phosphor.
This is attributed to the fact that both of the half width of the emission spectrum of the Eu-activated β-SiAlON phosphor and the half width of the emission spectrum of the Mn4+-activated fluoride complex phosphor are smaller than that of the emission spectrum the Ce-activated YAG phosphor. Specifically, the half width of the emission spectrum of the Eu-activated β-SiAlON phosphor is 55 nm or less. The half width of the Mn4+-activated fluoride complex phosphor is 10 nm or less.
As described above, the half width of the emission spectrum of the Eu-activated β-SiAlON phosphor is 55 nm or less and smaller than that of the emission spectrum of the Ce-activated YAG yellow phosphor. However, a combination with a phosphor having an emission spectrum with a smaller half width enables a wider-color-gamut image display apparatus.
Japanese Unexamined Patent Application Publication No. 2011-142336 (publication date: Jul. 21, 2011) discloses a structure in which two types of phosphors, i.e., a quantum-dot phosphor that emits green light and a quantum-dot phosphor that emits red light, are combined with a blue LED. However, in the structure described in Japanese Unexamined Patent Application Publication No. 2011-142336, the quantum-dot phosphor that emits red light absorbs green light emitted from the quantum-dot phosphor that emits green light to reduce the luminous efficiency of a light-emitting device.
Japanese Unexamined Patent Application Publication No. 2012-163936 (publication date: Aug. 30, 2012), Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-519232 (publication date: May 23, 2013), and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-534042 (publication date: Aug. 29, 2013) each disclose a structure in which a quantum-dot phosphor and a Mn4+-activated fluorine complex phosphor serving as phosphors dispersed in a wavelength conversion member are used in combination. Unlike the quantum-dot phosphor, the Mn4+-activated fluorine complex phosphor does not absorb green light. Thus, the structure described in each of Japanese Unexamined Patent Application Publication No. 2012-163936, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-519232, and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2013-534042 may control the problem in which green light emitted from a phosphor that emits green light is absorbed by a phosphor that emits red light.